The invention relates to a method for providing at least one defined volume of a target constituent of a sample, in particular of a blood sample. The invention further relates to a metering capillary and a device for providing at least one defined volume of a target constituent of a sample. Such methods, metering capillaries and devices can be used in particular to obtain defined amounts of plasma from capillary blood.
Testing samples of body fluids, for example blood, is often an important part of medical diagnosis. Such testing can be carried out in hospitals and also in the point-of-care sector or in home monitoring. The invention described below is concerned particularly with blood samples, although other types of samples, in particular liquid samples and preferably body fluids, can also be tested analogously. Without limitation in respect of other possible types of samples, the invention is described below mainly with reference to blood samples.
The samples are generally provided for at least a medical and/or diagnostic use. In a diagnostic use, they can be tested, for example, in respect of at least one property, for example at least one parameter that can be measured physically and/or chemically or biochemically. For example, the samples can be subject to a qualitative and/or quantitative detection of at least one analyte, in particular of at least one metabolite. For this purpose, numerous detection methods are known from the prior art.
For example, it is possible to detect glucose, cholesterol, triglycerides, hemoglobin, urea, alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transferase (GGT), creatinine (CREA) or high-density lipoprotein cholesterol (HDLC), or combinations of the analytes mentioned or of other analytes. In addition, other properties of the blood can be determined, for example a proportion of corpuscular constituents (hematocrit value).
A problem of testing blood samples for example, but also other types of samples, is that these blood samples in many cases have to be worked up prior to further use. In particular, it is necessary for many uses to break blood samples down into their constituents and, for example, to separate blood plasma from corpuscular constituents of the blood sample. This separation of the blood sample into its constituents generally has to be carried out very carefully, since many measurements require a high level of purity, i.e., a high degree of separation, at the same time with exactly defined quantities of sample. In many cases, these strict requirements in terms of the precision of the sample preparation make it difficult or impossible for analyses to be carried out by untrained persons, since the sample preparation for meeting the stated requirements generally has to be performed by trained personnel. For example, there are at present no devices on the market, or only a small number of devices, with which the parameter HDLC can be measured. The main reason for this is that, because of the coagulation that occurs during the separation, this parameter can be measured only in plasma to which anticoagulants are added and using an exactly defined sample volume (for example 31±1.5 μl), that is to say only after very careful sample preparation.
A known method of obtaining plasma from blood samples, for example capillary plasma, is to obtain capillary blood and then carry out centrifugation. For example, blood emerging from an incision in a surface of the body can be collected by means of a capillary and then subjected to centrifugation.
Devices for collecting and centrifuging capillary blood are known in principle from the prior art. For example, U.S. Pat. No. 5,456,885 describes a tube for collecting, separating and dispensing a two-phase liquid. In the meantime, capillaries are also commercially obtainable which, after being filled, are broken at a predetermined break point in order to obtain an exact sample volume. Such capillaries are commercially available for example from Dr. Müller Gerätebau GmbH, D-01705 Freital, Germany, or are described, for example, in DE 295 20 918 U1. The broken capillary parts, with the amount of blood located therein, are then introduced into a sample vessel, for example a cup. Such sample vessels are then centrifuged in corresponding centrifuges. Alternatively, a sample of capillary plasma can also be obtained from capillary blood by directly collecting the capillary blood in the sample vessels. After the centrifugation, during which the corpuscular portion of the blood sample separates from the blood plasma, the desired quantity of excess plasma is pipetted off from the excess of blood plasma.
With this pipetting, however, there is generally the problem that this procedure has to be done extremely carefully, since there is a risk that, during the pipetting, the coagulum at the vessel base is touched and, as a result, non-plasma constituents are also pipetted. The plasma fraction can be contaminated in this way, as a consequence of which measurement values can be considerably influenced.
This risk of contamination of the plasma fraction can be reduced only by collecting unnecessarily large amounts of blood or of plasma. For example, about 5 to 7 times the amount of necessary plasma generally has to be collected as capillary blood, in order to be able to obtain the required amount of plasma without risk of contamination.
However, this removal of quite large amounts of blood causes considerable difficulties in itself. For example, amounts of approx. 31±1.5 μl of pure blood plasma are typically needed for conventional tests, for example for the quantitative detection of one or more of the abovementioned analytes. In the abovementioned methods, however, this assumes a high volume availability of a starting quantity of capillary blood, which is not always possible. For example, according to the above, approx. 150-200 μl of capillary blood would be needed for this purpose, which is often only achievable in practice with difficulty.
Capillaries with predetermined break points are also known from other prior art documents.
DE 101 06 362 A1 describes a device and a method for collecting aqueous liquid samples. A capillary and a closure element are used, which closure element, including the capillary segment surrounding it, can be broken off or broken open by means of a predetermined break point. Moreover, the capillary has, in its interior, a mixing element made of ferromagnetic material, and retaining elements which serve to retain the mixing element. However, these retaining elements, in combination with the ferromagnetic mixing element, have the effect that centrifugation of the capillary at high speeds can lead to damage to the capillary channel. As one possible design of a capillary end, a Luer cone among other things is described, i.e., a cone-shaped reduction of the external diameter in the area of the capillary end.
In practice, however, the devices and methods known from the prior art present many technical challenges and disadvantages. An important disadvantage lies in particular in the handling safety of the known devices. Thus, in particular, capillaries with predetermined break points have to be manually or automatically manipulated several times before a desired target constituent of the sample can be provided. This manipulation by its nature involves many shaking movements and repeated positioning of the capillary in a wide variety of positions and orientations, which can lead to distortion of the measurement result. This can lead to amounts of sample leaking out, to undesired mixing-together of constituents of the sample, or to metering artifacts.